Issues with source code access aside, your description is mostly wrong. These programs take a DNA profile as input- it's just that the DNA profile is mixed (i.e. from multiple people). It reporting no DNA would be nonsensical. Figuring out exactly how many people are in a mixture isn't quite nailed down statistically (last I knew of), but it's usually pretty clear for up to 4 or so people.
Yes, you could run different models and get different probabilities. For example, the likelihood that the sample is a mixture of the suspect, the victim, and some unknown person vs victim and two unknown people compared to saying the victim isn't in the sample. However, the specification of those models is part of the trial process.
And the output probabilities (at least when being used to determine guilt) are usually quite high, orders of magnitude higher than 90% or even 99.99%.
My point is that the science behind these calculations is well developed- validation studies get published all the time. Whether or not the specific software has errors (or isn't coded exactly as modeled) is an entirely different matter, but it still isn't all that likely. All of these cases rely on expert witnesses anyway- it's not the prosecutor pressing some buttons and printing a report.
There is far more concerning quackery that gets used in forensics- bite marks, hair matching, etc.
Yes, you could run different models and get different probabilities. For example, the likelihood that the sample is a mixture of the suspect, the victim, and some unknown person vs victim and two unknown people compared to saying the victim isn't in the sample. However, the specification of those models is part of the trial process.
And the output probabilities (at least when being used to determine guilt) are usually quite high, orders of magnitude higher than 90% or even 99.99%.
My point is that the science behind these calculations is well developed- validation studies get published all the time. Whether or not the specific software has errors (or isn't coded exactly as modeled) is an entirely different matter, but it still isn't all that likely. All of these cases rely on expert witnesses anyway- it's not the prosecutor pressing some buttons and printing a report.
There is far more concerning quackery that gets used in forensics- bite marks, hair matching, etc.